1. Field of the Invention
This invention relates to lasers, and more particularly to semiconductor lasers which employ optical gratings.
2. Description of the Related Art
Semiconductor lasers are known which produce lasing action within the confines of a semiconductor chip. No external mirrors are necessary in general, since the high reflectivity from the refractive index differential at the semiconductor-air interface is normally sufficient. Since the chips are cleaved along natural crystalline planes, the parallelism of the reflective surfaces is assured, and further polishing of the optical surfaces may be unnecessary. A typical semiconductor laser is illustrated in very simplified form in FIG. 1. It consists of a thin layer of semiconductor material 2, generally about 1 micron deep and about 200 microns long. The device may be formed from semiconductors such as GaAs or InP. A gain medium 4 is defined by current injection to an internal pn junction formed by dopants, or by optical stimulation. In response to threshold stimulation, a laser beam 6 is emitted from the output side of the device. A general reference on semiconductor lasers which may be designed to operate in either pulsed or continuous wave modes, is provided by H. C. Casey and M. B. Parish, "Heterostructure Lasers", Academic Press, 1978.
One form of semiconductor laser, illustrated in FIG. 2, uses an optical spatial grating in the form of a distributed Bragg reflector 8 as the reflecting element at one end of the laser. The partial reflector at the output end of the laser is provided either by the semiconductor/air interface, or by another distributed Bragg reflector 10. Alternately, a distributed feedback grating may be used which spans the entire lasing cavity, thereby providing a more stable configuration. The application of distributed Bragg reflector and distributed feedback gratings to semiconductor lasers is discussed in the Casey and Parish reference mentioned above, and in U.S. Pat. No. 4,464,762 to Furuya. The periodicity of the gratings is selected to fix the output laser beam at a desired wavelength. For example, for GaAs having a wavelength of 860 nm, the periodicity would be spaced approximately 130 nm for first order operation.
A principal limitation of the semiconductor lasers discussed thus far, whether or not they employ optical gratings, is that they generally operate in a "stable resonator" mode. This means that the active resonation tends to be concentrated towards the center of the device, rather than spreading out uniformly through the resonation cavity. This phenomenon, known as filamence, is associated with instabilities in the laser media. Also, these devices must often lase in multiple longitudinal (or spectral) modes. If more power is desired, the device can be made larger, but this increases the multi-mode operation problem. The beams are also subject to undesirable nonlinear effects such as self-focusing, beam breakup, beam fanning and filamentation which can cause an incoherent beam. Furthermore, when reflection takes place at or very near the surface facets of the chip, a high energy density may structurally damage areas on the end facets. This can result in a significant degradation of the beam if the structural damage is substantial.
An "unstable resonator" laser has been proposed in which the resonation uniformly fills the lasing cavity, optical energy in the laser. In this approach, one or both of the end facets of the laser chip are physically worked into a curve, as opposed to the normal plane facet structure. As originally proposed, the curvature was formed by a physical grinding process, discussed in Bogatov et al., Soviet Journal of Quantum Electronics, Vol. 10, pp. 620-622, May 1980. The curvature is designed so that, upon repeated reflection from each end of the laser, the light spreads out and fills the gain medium, rather than being concentrated down the middle. A more refined manufacturing process, in which the desired curvature is etched by photolithography techniques, is discussed by Richard R. Craig, one of the present inventors, Craig et al., Electronics Letters, Vol. 21, No. 2, January 1985. Unstable optical resonators are also discussed in A. E. Siegman, Proceedings of the IEEE, Vol. 53, pp. 277-287, March 1965.
While the unstable resonator approach makes more efficient use of the lasing medium and is capable of extracting a greater energy level from the same laser device, it also has some significant drawbacks. The process of either grinding or etching the end facets of the laser to the desired curvatures is difficult to precisely control; crystalline structures naturally tend to cleave along straight lines. The grinding or etching process can damage the surfaces of the end facets, and can also introduce impurity absorption. Also, these devices can be subject to mode-hopping and chirping problems common to any diode laser that is not spectrally selective and cannot prevent transverse oscillations. This is a problem in large gain regions which might oscillate off of stray reflections not along the optic axis. Accordingly, while the available unstable resonator techniques provide at least a potential improvement in beam quality and efficiency, they introduce several disadvantages which are not found with other semiconductor lasers.